Charge pumps are generally a part of a phase lock loop system and are used in transmitter/receivers of wireless communication systems in conjunction with a voltage controlled oscillator to speed up the change from an existing frequency of operation to a different frequency of operation.
A typical phase lock loop system of the type used in wireless communication is shown in FIG. 1 and includes a reference frequency oscillator 1 which is a very accurate frequency source. The frequency emitted by the oscillator 1 is divided down by a predetermined factor N in a divider circuit 3 and fed to one input of a comparator 5. A voltage controlled oscillator (VCO) 7 having a known tuning or frequency output range, the output frequency of which is controlled by a control voltage applied thereto, provides an output frequency which is divided down by a predetermined factor M in a divider 9 and fed to the other input of comparator 5. The comparator 5 provides an output error signal indicative of the difference and direction of the difference in frequency output between the reference oscillator 1 and the VCO 7. The output frequency of the VCO 7 is the frequency at which the associated transmitter/receiver operates as is well known. The error signal is applied to a charge pump 11 which provides a charge onto a low pass or loop filter 13 composed of a capacitor 27 to ground with a series connected resistor 29 and capacitor 25 in parallel therewith. The filter 13 is coupled to the VCO 7 to control the output frequency of the VCO and drive the output of the VCO to the desired and correct frequency.
In operation, it is often necessary to change the frequency of operation of the VCO 7. The amount of frequency adjustment within the frequency output range of the VCO can vary from very small up to the entire limit of the output range. The greater the amount of frequency adjustment required, the greater is the amount of current (translated as voltage) required to make the frequency adjustment. The speed of this change of frequency is generally controlled by the use of a charge pump which is generally composed of a pair of current sources 15, 17. The error signal will be small from the comparator 5 to the charge pump 11 when the frequency of operation remains unchanged or the charge pump will supply small changes in current for small frequency adjustments via the current sources 15, 17. However, for a rapid change in operating frequency, a large amount of current must be sinked or sourced into the loop which results from the large error signal supplied by the comparator 5. In the prior art, this has been accomplished by applying an even larger current from a separate current source 19 in addition to the current from the current sources 15 and 17 with a switch (not shown) being enabled to create a path for a predetermined limited time under control of a timer 23 to the capacitor 25 of the filter 13 in the loop to provide additional large current for a short time period to provide the rapid change in operating frequency of the VCO 7. This speed up circuit effectively opens the bandwidth of the low pass or loop filter 13 to improve speed-up time when the synthesizer switches from one frequency to another. The limitation is that when the switch (not shown) is enabled in the integral leg of the loop filter, the charge pump blindly delivers a charge to the capacitor with little control over the amount of current being delivered. This can and usually does result in significant undershoot or overshoot of the new target VCO frequency. Also, the charge pump delivers current only when there is a phase difference in the phase frequency detector. Although the lock times are better than without speed-up mode, the switching requirements in the Global System of Mobile Communications (GSM), the most used digital wireless standard in the world, and other wireless systems are such as to preclude prior art approaches to speed-up mode circuits for conventional integer N synthesizers or fractional N synthesizers.